The present invention relates in general to integrated circuits and in particular to a method and a device for determining the ratio between an actual value of an RC time constant in an integrated circuit or chip and a set or desired value of the RC time constant.
Resistor-capacitor (“RC”) circuit networks or elements (hereinafter “RC network”) that have at least one ohmic resistor and at least one capacitor connected together may be used in many applications. Examples include timing elements or filters. One parameter describing the response of such an RC network is its time constant which may be derived from the product of the capacitance value of the one or more capacitive components and the resistance value of the one or more ohmic components. Depending on fabrication-related fluctuations and/or the operating conditions associated with a circuit that includes an RC network, the actual or measured value of the time constant can fluctuate significantly relative to an expected or desired fabrication-specific set value of the time constant. For example, variances (sigma values) of approximately 6% of the actual value from the set value may commonly occur.
As a result of these fluctuations, it may be desirable to determine the value of the RC time constant before using a circuit that includes such an RC network to be able to calibrate the circuit. Depending on the intended application, the calibration may be performed on a one-time basis after fabrication of the circuit that includes the RC network, or may be performed during circuit operation in conjunction with changing conditions of use.
FIG. 1 is a schematic diagram of a prior art low-pass filter that comprises an RC network 10. The network 10 may comprise, for example, two resistors 12, 14 connected in series with a parallel connection of a plurality of capacitors 16–22. Two of the capacitors 20, 22 can each be selectively connected or disconnected into or out of the network 10 by corresponding switches 24, 26, to thereby calibrate the RC network 10.
In known methods for determining an RC time constant, charging and discharging processes of the capacitor(s) of an RC network may be implemented, where the assumption may be made that the RC time constant of any additional RC networks in a circuit may respond in terms of a set value in a manner analogous to that of the measured RC network. The actual value of the time constant can be determined based on the time required to charge the capacitor of the reference RC network from a lower reference voltage value up to an upper reference voltage value, or to discharge the capacitor from an upper reference voltage value to a lower reference voltage value, together with the knowledge of the upper and lower reference voltage values. The accuracy with which the time constant can be determined by such a method may be a function of the accuracy with which the required time references and reference voltages are provided or determined. The charge or discharge times determined may be quantified by the multiple of clock pulse periods of a clock signal serving as the time reference. The accuracy in determining the time constant may increase with the clock rate of the clock signal and with an increasing duration of the charging and discharging processes. Extending the charge and discharge times can be achieved by increasing the time constant of the reference RC network, which may undesirably require a physically larger resistor and/or a larger capacitor, and thus a larger surface area on the chip or integrated circuit.
What is needed is a method and device for determining the ratio between the actual value of the RC time constant of at least one RC network in an integrated circuit and a set value of the RC time constant, where a relatively accurate determination of this ratio may be provided with a small surface area requirement on the integrated circuit and with low circuit complexity.